One outbound link
from your Tumblr sites
to your money site
total.
It's generally a waste to keep posting on your Tumblrs after the initial setup. Build out something decently nice that will pass a manual spam review from the Tumblr moderators so that your domain doesn't get removed for looking like spam (because that would cause your backlink to disappear). Other than that, add your backlink (see footnote) and move on.
Making more posts over time isn't really going to help make that link any stronger, as far as Google's algorithms are concerned. Once you have the initial setup done, you're going to get the most bang for your buck by moving on to building the next one and forgetting about going back and micromanaging the old ones. If you want to rank for real, worthwhile keywords, you're building tens if not hundreds of these, to both specific landing pages and generally some to your homepage as well. It's way too much overhead for minimal or no additional value to keep adding content to and checking on old Tumblrs.
Footnote: Make sure that you set up your backlinks so that they are d0follow. Easiest way to do that on Tumblr is by putting them in the description. Putting links into regular Tumblr posts ends up being implemented as a redirect. @BassTrackerBoats' video guide on that is pretty straightforward about showing how to do this:
https://www.blackhatworld.com/seo/using-tumblrs-to-get-backlinks-my-worst-video-ever-by-far.843506/